The present invention relates to an actuator assembly and, more particularly, to such an assembly which provides translational movement of a ball screw between an extended and a retracted position.
Actuators, in which a helically grooved ball screw is mounted for reciprocal axial movement in a ball nut, are used in many environments in which translational movement of a device is desired. The ball nut is secured against axial movement and is rotated to cause corresponding axial movement of the ball screw between an extended and retracted position. Actuators are used with many devices, such as, for example, gate valves or the like, which are attached to the ball screw for translational movement therewith. The ball nut can be rotated by fluid pressure as disclosed in U.S. Pat. No. 4,691,893; by an electrical motor as disclosed in U.S. Pat. No. 5,195,721; or in any other conventional manner.
In some of these systems, an inner race is driven, via a reduction gearing system, by the power source and is coupled to a driver by a wrap spring which functions as a clutch. The driver is connected, by a gear and pinion, to the ball nut for rotating same. A low power activating device, such as an electrical solenoid, or switch, is usually provided which, when activated by a relatively low power signal, engages a control module that latches to the driver and therefore couples the driver to the inner race by causing the wrap spring to couple the inner race to the driver. This, in turn, causes the ball nut to rotate and the ball screw to extend from the ball nut and move to its fully extended position. A stem is usually connected to the ball screw for receiving a gate valve, or the like, which moves with the ball screw.
When it is desired to retract the ball screw, the low power activating device is deactivated, causing the control module to release the wrap spring from its coupled position and thus release the connection between the rotating inner race and the driver. A helical spring is provided which is compressed during the above extended position of the ball screw and which urges the ball screw back to its retracted position when the above connection between the inner race and the driver is released.
However it is particularly difficult to control the large forces required to retract and extend the ball screw against high pressure with a very small force required to actuate the control module which latches to the driver. A main reason for this difficulty is that high gain "amplifiers" of this type tend to be unstable and thus unreliable. Also in these arrangements, multiple gears are usually provided between the control module and the ball nut. Therefore, when there is a malfunction of the coupling mechanism for whatever reason, a catastrophe can occur since several gears could become jammed.
Also in these types of arrangements, when the ball screw is in its extended position, after the motor has been turned off, and the control module been be de-energized, the gears connecting the motor are subjected to a reverse, or "backward" torque which can cause damage. Further, in some of these designs the torque from the rotating ball nut is transferred to the stem 78 which could compromise the operation of a component, such as a valve, or the like, connected to the stem.
Therefore, what is needed is an actuator assembly that controls the large forces required to retract and extend the ball screw with a control module that latches the system in response to a holding signal from an activating device that consumes a relatively low amount of power for a relatively long time. Also needed is an actuator assembly of the above type that is stable and reliable and, as such, resists vibration and environmental factors which tend to "trip" the unit. Further an actuator assembly of the above type is needed in which minimal gearing is provided between the control module and the ball nut to eliminate the possibility of multiple gears becoming jammed if a malfunction occurs. Also needed is an actuator assembly of the above type in which backward rotation of the gears of the drive assembly is prevented and in which the stem, and therefore any valve, or the like, connected to the stem, is not subject to any torque from the rotating ball nut.